The recovery of uranium, vanadium and equivalent elements by in situ leaching operations is described in a number of U.S. patents. In this regard, see, for example, U.S. Pat. No. 2,896,930; U.S. Pat. No. 3,309,140 and U.S. Pat. No. 4,103,963. Typically, in such processes, wells are drilled in accordance with a predetermined selected pattern into an underground deposit of uranium mineral. After completion of the wells, a leach solution is flowed between the wells to dissolve the uranium, vanadium and other metal values in the leach solution. The leach solution is then carried to the surface where the pregnant lixiviant is treated to recover the mineral values therefrom. The leaching process obviously is continued until the concentration of metal values reaches a sufficiently low point where it becomes uneconomical to recover the mineral values from the solution. At this point, soluble uranium remains in the formation at concentration which is environmentally unacceptable.
One approach to reducing the uranium levels in such formations is to continue to remove formation water from the deposit while simultaneously injecting fresh or treated waters. The water removed from the underground deposit can be treated above ground by a technique such as reverse osmosis which in addition to removing a soluble uranium, also removes other contaminants associated with the mining process. Experience has shown that many of the contaminants in such formation waters are quickly reduced to levels which are environmentally acceptable. Solubilization of the uranium, however, in the deposit persists long after other contaminants have been removed. This phenomena apparently is the result of a continued slow diffusion of previously oxidized uranium minerals, or solubilization from minerals which continue to be leached by residual oxidant after the intrusion of fresh water into the formation.
An alternate technique for restoring a formation which has undergone an in situ leaching operation involves the injection into the formation of a restoration fluid which contains reducing agents capable of reducing oxidized mineral and/or metal values to their reduced insoluble state. In this regard, see, for example, U.S. Pat. No. 4,234,231. Since huge excesses of oxidant are used during the mining process, relatively large amounts of reductant will be required during the restoration process, particularly in view of the fact that the reduction process is not selective. Moreover, residual oxygen from the mining process undoubtedly will remain in the resevoir indefinitely, thereby providing a source of oxidant for reoxidation of the reduced uranium. Finally, with increasing environmental regulations and demands, the achievement of even lower levels of solubilized uranium in the restored formation is needed.